Topcoat compositions applied to automotive exterior panels are required to form a topcoat film with excellent coating film properties, such as distinctness of image (DOI) and water resistance.
The topcoat film is usually a multilayer coating film comprising a base coating and a clear coating.
The base coating of a topcoat film is usually formed by using a color base coating composition containing a pigment such as an effect pigment, a coloring pigment, or the like. By applying a transparent, clear coating composition over the base coating to form a multilayer coating film, the resulting topcoat film can be provided with an excellent design derived from the base coating, and also with an excellent gloss and smoothness derived from the clear coating.
Many organic solvent-based base coating compositions have been used as base coating compositions. However, when such a base coating composition is used, an organic solvent vaporizes when the coating is baked, thereby causing an environmental problem, i.e., pollution. Recently, the use of less pollutant aqueous base coating compositions has been increasing. As such aqueous base coating compositions, compositions prepared by incorporating a melamine resin as a crosslinking agent into an acrylic, polyester or like resin having a functional group such as hydroxy, carboxy, etc., have often been used.
Many water-soluble melamine resins that have a comparatively low molecular weight and are highly compatible with water have been used as melamine resins in such aqueous base coating compositions. Recently, the use of hydrophobic melamine resins having a comparatively high molecular weight has been considered. Compared with aqueous base coating compositions comprising a water-soluble melamine resin, aqueous base coating compositions comprising a hydrophobic melamine resin can form a coating film with excellent coating properties, such as water resistance. However, hydrophobic melamine resins are poorly compatible with water, which is the coating composition medium. Therefore, when such a hydrophobic melamine resin is used, the obtained coating film may have poor DOI. Furthermore, when the composition comprises an effect pigment such as aluminum flakes, mica, etc., metallic unevenness may occur in the film.
Japanese Unexamined Patent Publication No. 2002-308993 discloses an aqueous resin dispersion with excellent water dispersion stability, comprising a reaction product obtained by subjecting an acrylic resin, a hydrophobic melamine resin, and a polyester resin to a heat treatment. The publication describes that this dispersion can form a coating film with an excellent coating appearance. However, since production of the aqueous resin dispersion requires a heating step, it is disadvantageous in terms of energy savings. Furthermore, when the aqueous dispersion is used, metallic unevenness may occur in the obtained coating film.
U.S. Pat. No. 4,900,774 discloses an aqueous coating composition comprising a crosslinking agent prepared by dispersing a hydrophobic melamine resin in water in the presence of a water-soluble resin. The patent describes that this composition has excellent storage stability and the use of the composition enables appropriate coating work to be performed in a wide range of humidities, without causing defects such as sagging and unevenness. However, coating films formed using the aqueous coating composition may have poor DOI and poor water resistance.
U.S. Pat. No. 5,539,049 discloses an aqueous resin dispersion prepared by dispersing a graft resin and a hydrophobic melamine resin in an aqueous medium, the graft resin having a hydrophilic side chain moiety with a weight average molecular weight of 5,000 to 50,000 and an acid value of at least 20. The patent describes that this dispersion is excellent in terms of storage stability, thermal stability, mechanical stability, etc. However, when an aqueous base coating composition comprising this aqueous resin dispersion is used, the obtained coating film may have poor DOI and poor water resistance and metallic unevenness may occur in the coating film.
Examples of methods for forming a multilayer topcoat film comprising a base coating and a clear coating include a two-coat one-bake method comprising applying a clear coating composition over an uncured base coating, and baking to cure the base coating and the clear coating at the same time; and a two-coat two-bake method comprising baking to cure a base coating, then applying a clear coating composition over the base coating, and baking to cure the clear coating. Currently, the two-coat one-bake method is widely used in view of energy savings, etc. However, according to the two-coat one-bake method, since a clear coating composition is applied over an uncured base coating, a mixed layer is generated at the interface between the base coating and clear coating, so that the obtained coating film may have poor DOI. Another problem is that metallic unevenness is likely to occur in the coating film.
Japanese Unexamined Patent Publication No. 2004-73956 discloses using an aqueous base coating composition containing a specific tertiary amine, thus providing a multilayer coating film with excellent appearance and water resistance. However, when this method is used, metallic unevenness may occur in the obtained coating film.